It is common to apply differentially coded pulses to an encoding circuit for recording or for direct transmission. Such a circuit typically includes first and second input amplifiers and first and second output comparators. Such coded pulses are applied to inputs to the amplifiers resulting in corresponding changes in the amplifier output voltages.
The input circuit must exhibit high impedance when pulses are applied. Typically, 40 megohm resistors are necessary to achieve the requisite high impedance. The problem is that such resistors cannot be implemented on chip, and they are too expensive to implement off chip. Although the requisite high impedance can be achieved by the use of a current source, such sources are process sensitive. Accordingly, such an approach is not attractive.